Applying a film to a body

ABSTRACT

A method of applying a thin film to a body comprising exposing the body to pulsed-gas cold-plasma polymerization of an unsaturated-carboxylic acid monomer thereby forming a polymer film on a surface of the body.

This invention relates to a method of applying a fluoropolymer film to abody and to bodies so treated.

Oleophobic or superhydrophobic surfaces are desired for a number ofapplications. The invention arises out of investigations of thephenomenon of surfaces with lower energy than ptfe(polytetrafluoroethylene) by taking advantage of the effect arising fromattachment CF₃ groups to a variety of materials.

The invention may be applicable to thin films usable in polymeric filtermedia and to cold plasma treatments to create low energy surfaces uponlow-cost thermoplastics and natural media, and to the functionalisationof fluorinated polymers such as PTFE and PVDF (polyvinylidenedifluoride). This specification discusses a plasma procedure leading toa thin film of perfluoroalkyl groups upon a substrate, which willexhibit superhydrophobicity or oleophobicity. By this we mean that thesurface will repel liquid with surface energies as low as that ofacetone and alcohol.

The controlled deposition of many plasma polymers has been examined andthe ratio of CF₂ to CF₃ is documented in terms of monomer type, plasmapower levels and proximity to the glow region. We are now describing anew method for creating surfaces with greater coverage of functionalgroups which offers a novel approach to the creation of polymer surfacesby pulsed gas introduction of the plasma.

According to the present invention, a method of applying a fluoropolymerfilm to a porous or microporous or other body, comprises exposing thebody to cold plasma polymerisation using a pulsed gas regime to formeither (i) an adherent layer of unsaturated carboxylic (e.g. acrylic)acid polymer on the surface and then derivatising the polymer to attacha perfluoroalkyl group terminating in —CF₃ trifluoromethyl. Acombination of electrical and gas pulsing may be used.

Preferably, the cold method of applying a fluoropolymer film accordingto 1 and 2 wherein the cold plasma polymerisation uses an unsaturatedcarboxylic acid.

The “gas on” and “gas off” times are preferably from 0.1 microsecond to10 seconds.

The pulsed gas may be oxygen, or may be a noble or inert gas or H₂, N₂or CO₂. Alternatively, acrylic acid polymer precursor may be pulseddirectly without a process gas.

The body may be a film (not necessarily microporous) or of othergeometry that allows coating by plasma polymerisation to a standard ofconsistency adequate for the end use.

The method may be stopped at any stage, when the applied film iscontinuous and impervious or at an earlier stage, when it is to agreater or lesser extent still apertured, i.e. has not yet completelyfilled in the underlying pores of the body. The pore size of thefinished product can be set to any desired value by ceasing the methodafter an appropriate duration.

The plasma power is preferably 1W to 100W, more preferably 1.5W to 7W.

The invention extends to the body with the thus-applied film. Thesubstrate material of the body may be carbonaceous (e.g. a naturalmaterial such as cellulose, collagen or alginate, e.g. linen),synthetic, ceramic or metallic or a combination of these.

Electrical pulsing of the radio frequency supply to the plasma is known.This technique can endure a more rapid deposition and greater coverageof the substrate surface by the plasma polymer. We have utilised theplasma polymerisation of acrylic acid, which again is known but using apulsed gas regime and clearly there are many other possible unsaturatedcarboxylic acids available as monomers. It is believed that suchfunctionalities impart a degree of biocompatibility to substrates andallow of call culture experiments to be undertaken successfully uponsuch a surface even with difficult an sensitive cell lines.

By virtue of a derivatisation stage, the acid group may be reacted witha range of materials, for example perfluoralkylamines, to yield asurface rich in perfluoroalkylamide groups. In this way the surfacewould predominate in CF₃ functions. Additionally the use of fluorinatedsurfactants will similarly generate a surface film of lower energy thanptfe and find application in for example the packaging market whereoleophobic materials are desirable.

In the packaging market, there is a need for oleophobic venting filmswhere the contents of a vessel or a package may require the release ofdifferential pressure. Such pressure differentials may arise fromexpansion or contraction of the container or the liquid contents, withchanges in the ambient temperature or pressure. The liquid contents mustbe retained without leakage and so porous venting aids are used. Inthose situations where liquids of low surface tension are involved e.g.surfactants, detergents, or organic solvents, then conventional porousptfe materials are not as efficient. The surface energy of suchmaterials is of the order of 18 to 20 dynes/cm at 20° C. and the energyof a CF₃ surface is less at perhaps 6 dynes/cm, and can be influenced bythe plasma conditions used for the deposition. It is also known that thesubstrate morphology can influence the value of the contact angle sincesurfaces of a certain roughness can lead to composite angels. Thesurface which has the greatest number of CF₃ groups packed together willhave the lowest surface energy.

Products having superior (high density) surface coverage, rapidlydeposited, may arise from gas pulsing alone or in combination with R.F.pulsing. Such materials have application in filtration, chromatography,medical device and laboratory ware. For example low cost thermoplasticscould be coated using perfluorocarbon monomers to afford ptfe-likeproperties.

The body or substrate upon which the superhydrophobic layer is attachedmay be a carbonaceous polymer, e.g. a fluoropolymer such as ptfe,optionally itself a film, which may be porous or microporous. Theprocess can also be applied to other polymers such as polyethylene and arange of other materials used for the biocompatible properties conferredby the acidic groups. Additionally by conversion to functionalitiesterminating in perfluoroalkyl groups the superhydrophobic properties ofthe closely spaced CF₃ groups can be utilsed. In certain applications itis commercially attractive to change the surface properties of low costmaterials such that they become superhydrophobic. For example celluloseof polyurethane foam are used for their absorbent nature in wounddressings and incontinence and other sanitary products. By virtue of thehydrophobic layer being present in the wicking effect can be directedand the flow of exudate or moisture constrained. Similarly for fluidswith lower surface tension a superhydrophobic or oleophobic layer wouldoffer the same mechanism.

A specific embodiment of the invention will now be described by way ofexample with reference to the accompanying drawings (all graphs), inwhich:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows C(Is) XPS peak fit for 2 W continuous wave plasma polymerof acrylic acid.

FIGS. 2a and 2 b show continuous wave plasma polymerisation of acrylicacid as a function of power: (a) Q1s) XPS spectra; and (b) O/C ratio andpercentage retention of acid functionality.

FIGS. 3a and 3 b show C(Is) XPS spectra for electrically pulsed plasmapolymerisation of acrylic acid: (a) as a function of T_(on) (T_(off)=4ms and P_(p)=5 W); and (b) as a function of T_(off) (T_(on)=175 μs andP_(p)=5 W).

FIGS. 4a and 4 b show dependence on average power of: (a) oxygen:carbonratios; and (b) percentage acid group incorporation for continuous wave;and electrically pulsed plasma polymerisation of acrylic acid as afunction of T_(on) (T_(off)=4 ms and P_(p)=5 W and 70 W) and T_(off)(T_(on)=175 μs and P_(p)=5 W).

FIG. 5 shows variation in the O/C ratio and percentage acid groupincorporation during electrical and gas pulsed plasma polymerisation ofacrylic acid using different gases (T_(on)=175 μs T_(off)=4 ms andP_(p)=5 W).

FIGS. 6a and 6b show electrical and gas pulsed plasma polymerisation ofacrylic acid using oxygen as a function of T_(on) (T_(off)=4 ms andP_(p)=5 W):(a) C(Is) XPS spectra; and (b) O/C ratio and percentage ofacid group retention.

FIGS. 7a and 7 b show 2 W continuous wave plasma polymerisation ofacrylic acid as a function of oxygen pressure: (a) C(Is) XPS spectra;and (b) O/C ratio and percentage retention of acid functionality.

FIGS. 8a and 8 b show electrical and gas pulsed plasma polymerisation ofacrylic acid with oxygen as a function of T_(off) (T_(on)=175 μs andP_(p)=5 W):(a) C(Is) XPS spectra; and (b) O/C ratio and percentage ofacid group retention.

FIGS. 9a and 9 b show ATR-IR spectra of: (a) acrylic acid monomer; and(b) Electrical and gas pulsed plasma polymer of acrylic acid, usingoxygen, deposited on polyethylene (T_(on)=175 μs _(off)=4 ms and P_(p)=5W), and

FIG. 10 shows XPS spectra of plasma polymerisation of acrylic acid underCW, electrically pulsed and electrically-and-gas pulsed plasmaconditions.

All plasma polymerisations were performed in an electrodelesscylindrical glass reactor (50 mm diameter) enclosed in a Faraday cage.The reactor was pumped by a two stage rotary pump (Edwards E2M2) via aliquid nitrogen cold trap (base pressure of 5×10⁻³ mbar). Power wassupplied from a 13.56 MHz source to a copper coil (10 turns) woundaround the plasma chamber via an L-C matching unit and power meter.

Prior to each experiment, the reactor was scrubbed clean with detergent,rinsed with isopropyl alcohol, oven dried and further cleaned with a 50W air plasma ignited at a pressure of 0.2 mbar for 30 minutes. A glassslide which has been washed in detergent, then ultrasonically cleaned in1:1 cyclohexane and IPA for one hours, was positioned at the centre ofthe copper coils and the system pumped back down to base pressure.

Before polymerisation the acrylic acid (Aldrich 99%) was subject toseveral freeze thaw cycles and used without further purification. Themonomer vapour was admitted via a needle valve (Edwards LV 1OK) to apressure of 0.2 mbar for 2 minutes prior to ignition of the plasma. Ifgas was also to be added it was introduced via a needle valve (EdwardsLV 1OK) to the required pressure. For gas pulsing experiments, gas waspulsed into the system by a gas pulsing valve (General Valve Corporation91-110-900) driven by a pulse driver (General Valve Corporation IotaOne). Both continuous wave and pulsed plasma polymerisations wereperformed for 10 minutes.

For pulsed plasma experiments the R.F. generator was modulated by pulseswith a 5 V amplitude supplied by the pulse driver used to drive the gaspulsing valve. Pulse outputs from both the pulse generator and the R.F.generator were monitored by an oscilloscope (Hitachi V-252). Forexperiments involving both gas and electrical pulsing the pulse driverwas used to simultaneously supply the gas pulsing valve and the R.F.generator. Thus the gas pulsing valve was open while the plasma was on.

Upon termination of the plasma, the reactor system was flushed withmonomer and gas (where applicable) for a further 2 minutes, and thenvented to air. Samples were then immediately removed from the reactorand affixed to the probe tips using double sided adhesive tape foranalysis.

A Vacuum Generators ESCA Lab Mk 5 fitted with an unmonochromated X-raysource (Mg Kα=1253.6 eV) was used for chemical characterisation of thedeposited films. Ionised core electrons were collected by a concentrichemispherical analyser (CHA) operating in a constant analyser energymode (CAE=20 eV). Instrumentally determined sensitivity factors for unitstoichiometery were taken as C(Is):0(Is):N0s ):Si(2P)=1.00:0.39:0.65:1.00. The absence of any Si(2p) XPS featurefollowing plasma polymerisation was indicative of complete coverage ofthe glass substrate. A Marquardt minimisation computer program was usedto fit peaks with a Gaussian shape and equal full width at half-maximum(FWHM).

RESULTS

FIG. 1 shows the C(Is) envelope obtained by XPS analysis of acrylic acidplasma polymer. Five different carbon functionalities were fitted: C_(x) H_(y) (285 eV), C CO₂ (285.7 eV), C O (286.6 eV), O—C O/C=O (287.9eV), and CO₂ (289.0 eV). The hydrocarbon peak was used as a referenceoffset. The oxygen:carbon ratio was calculated by dividing the oxygenpeak area (after the sensitivity factor had been taken into account) bythe carbon peak area. The relative amounts of acidic carbon atomretention was compared by calculating the percentage of CO₂functionality relative to the total C(1s) area.

Continuous wave experiments were carried out at discharge power between1.5 and 7 W, FIG. 2. As reported in earlier studies greater oxygenincorporation and acid group retention is achieved on decreasing thepower of the discharge. The best results were found at a discharge powerof 1.5 W which gave an O/C ratio of 0.52±0.02 and an acid groupretention of 18%±1.

This is considerably less than the oxygen:carbon ration of 0.67 and anacid group of 33% anticipated from the monomer structure. Variouselectrical pulse plasma polymerisation experiments were investigated inan attempt to improve retention of the monomer structure, FIGS. 3 and 4.It was found that decreasing the average power of a pulse modulatedplasma discharge, by systematically reducing the plasma ontime orincreasing the time-off, enhances oxygen incorporation and acid groupretention in the plasma polymer. Both the oxygen:carbon ratio and thelevel of acid group retention found under the lowest average powerconditions are significantly greater than found for the continuous waveexperiments. The O/C ratio at the lowest average power was found to be0.72±0.03 and the acid group retention was 30%±1.

Pulsed addition of various gases was found to increase O/C ratios, FIG.5. The percentage acid group showed less variation except when the gasused was oxygen. A large increase, well above monomer values, in boththe O/C ratio and acid group retention is evident when oxygen is addedto the plasma.

Gas and electric pulse time-on greatly influence the plasma polymercomposition, FIG. 6; at gas and electrical pulse on times below approx.130 μs, the electrical power of the plasma is dominant. The effect ofoxygen in the system is negligible. Decreasing the time-on increases thefunctionality of the plasma polymer. Beyond 140 μs the oxygen partialpressure in the system becomes non trivial. The composition of the thinfilms produced are altered markedly by this increase in the partialpressure of oxygen reaching a maximum at approx. 175 μs. Under theseconditions of the oxygen:carbon ratio was 1.00±0.04 and the percentageacid group was 43%±2.

Continuous wave polymerisation in the presence of oxygen has a directinfluence on the functionalisation of films formed, FIG. 7. Increasingthe oxygen content in a low power continuous wave plasma increases theO/C ratio and the percentage acid group retention. The effect is lesspronounced than for pulsed modulated systems.

Increasing the plasma and gas time-off for the electrical and gas pulsedplasma polymerisation of acrylic acid using oxygen decreases thefunctionalisation of the films produced, FIG. 8. This is opposite to thetrend reported above for the electrically pulsed polymerisation ofacrylic acid alone and it may be attributed to the decrease in oxygencontent of the plasma with increasing gas time-off.

The ATRAR spectrum of the acrylic acid monomer has the following peaks,FIG. 9a: O—H stretch (3300−2500 cm⁻¹), C—H stretch (2986−2881 cm⁻¹), C═Ostretch (1694 cm⁻¹), C═C stretch (1634 cm⁻¹), O—H bend (1431 cm⁻¹), C-Ostretch (1295−1236 cm⁻¹), C—H out-of-plane bend (974 cm⁻¹), O—Hout-of-plane bend (918 cm⁻¹), and ═CH₂ wagging (816 cm⁻¹). An ATR-IR ofthe plasma polymer deposited onto polyethylene, FIG. 9b, demonstrates alarge amount of oxygen functionalisation with the O—H bend and the C=Ostretches clearly evident.

To optimise the derivatisation of the poly(acrylic acid) or similarlayer with fluorinated surfactant, the reaction between a carboxylicacid (or e.g. ethylene oxide or styrene oxide) and a fluorinated aminemay be used. The fluorinated surfactant may be for example

Dupont FSD™, a commercially available fluorinated surfactant with aterminal CF₃ group, the opposite end possessing a cationic head based ona substituted ammonium ion, or

Hoechst AG 3658™

F₃C—(CF₂)_(n)—CH₂—CH₂—N⁺ (Alkyl)₃I.

Fluoroalkyl trialkyl ammonium salt.

Formation of the sodium salt of the poly(acrylic acid) PAA is followedby reaction with a solution of the fluorinatd surfactant, thecarboxylate anion and the cationic fluorosurfactant forming a salt withthe fluoro-chain (terminating in a CF₃ group) uppermost. e.g.

An alternative route involves a further cold plasma step using sulphurhexafluoride, SF₆. This reagent will yield CF₃ groups when reacted withcarboxylic acids or with esters.

A very high degree of functional group control has been achieved by thecombined pulsing techniques; see FIG. 10.

What is claimed is:
 1. A method of applying a film to a body comprisingexposing the body to pulsed-gas cold-plasma polymerization of anunsaturated-carboxylic acid monomer thereby forming a polymer film on asurface of the body.
 2. The method of claim 1, further comprisingderivitizing the polymer film with a fluoro-substituted group therebyproducing a fluoro-substituted film on the surface of the body.
 3. Themethod of claim 2, wherein the fluoro-substituted group comprises aterminal-trifluoromethyl group.
 4. The method of claim 2, wherein thefluoro-substituted group is a fluorinated surfactant.
 5. The method ofclaim 2, wherein the fluoro-substituted group is a perfluoroalkylamine.6. The method of claim 2, wherein the fluoro-substituted group is afluoroalkyl-trialkyl-ammonium salt.
 7. The method of claim 1, whereinthe body is porous or microporous.
 8. The method of claim 1, wherein theunsaturated-carboxylic acid monomer is acrylic acid.
 9. The method ofclaim 1, wherein a combination of electrical pulsing and gas pulsing isused.
 10. The method of claim 1, wherein both the gas-on and gas-offtimes are within the range of about 0.1 microseconds to about 10seconds.
 11. The method of claim 1, wherein the pulsed gas is oxygen.12. The method of claim 1, wherein the pulsed gas is a noble or inertgas or is hydrogen, nitrogen, or carbon dioxide.
 13. The method of claim1, wherein the unsaturated-carboxylic acid monomer is pulsed directlywithout a process gas.
 14. The method of claim 1, wherein the plasmapower applied is within the range of about 1 Watt to 100 Watts.
 15. Themethod of claim 1, wherein the plasma power applied is 1.5 Watts to 7Watts.